In general, operating speed becomes faster as a homojunction bipolar transistor size decreases. However, since an impurity concentration between emitter and base should be increased, enhancement of characteristics of the transistor based on a structure of such element is a difficult task.
There have been proposed heterojunction bipolar transistors to cope with the above disadvantage.
Such a heterojunction bipolar transistor has a structure in which the energy bandgap of an emitter is larger than that of a base. For this reason, the heterojunction bipolar transistor shows an improvement of the performance of element and various design effects in comparison to the homojunction bipolar transistor.
As a conventional homojunction silicon bipolar transistor, the prior art heterojunction bipolar transistors utilize a polysilicon thin film as both a base electrode and a impurity diffusion source for an emitter.
Thus, using Ge instead of Si on the base layer, a difference between the energy bandgap of the emitter and that of the base is made to increase an emitter implantation efficiency, then the base is grown to a high doping concentration ultra-thin film, thereby enhancing a current gain and a switching speed of the device.
Recently, according as the optimization and the miniaturization of the structure of element, various methods have been used to minimize several parasitic components such as base resistance at an active region of the element and parasitic capacitance between a collector and a base.
Examples of such various methods include trench isolation, local oxidation of silicon( "LOCOS"), selective epitaxial growth("SEG") of a SiGe base thin film, and selective epitaxial growth for a Si emitter and so on.
Using the methods described above, there have been developed a super self-aligned Si/SiGe heterojunction bipolar transistor which self-aligns the base and emitter to reduce a base parasitic resistance, or self-aligns both the base-emitter and collection-base.
Furthermore, in order to further reduce the base parasitic resistance resulting from the poly-silicon thin film forming a base electrode material, a manufacturing process using a metallic thin film, for example titanium silicide, instead of the polysilicon thin film has been developed.
The above local oxidation of silicon method has a shortcoming in that a bird's beak shaped thermal oxidation film is formed horizontally as a thickness of a vertical silicon thermal oxidation film, resulting in a limitation of geometric reduction of the element.
FIG. 1 shows an exemplary super selfaligned Si/SiGe heterojunction bipolar transistor which utilizes the selective epitaxial growth method for a SiGe base thin film without using the LOCOS method.
FIG. 1 shows a cross-sectional view of npn Si/SiGe heterojunction bipolar transistor formed by self-aligning the collector-base using the prior art selective base epitaxial growth, after the growth of a base thin film.
An n.sup.+ buried silicon collector layer (1-2) is first formed on a p-type silicon substrate (1--1), and an n-silicon collector layer (1-3) is then grown on the buried collector layer (1-2).
Subsequently, a collector junction portion (1-4) is formed by implanting an n-type impurity ion thereon, and then a trench is formed by a dry etching method to isolate elements, and, in turn, filling therein with Boron Phosphorous Silica Glass("BPSG") insulating thin film (1-5) made of boron B and phosphorous P. The BPSG insulating thin film (1-5) is then flattened under a high pressures.
In an ensuing step, an insulating film (1-6), a P.sup.+ polysilicon film (1-7), an insulating film (1-8) and a sidewall insulating film (1-9) are formed by depositing and etching methods as shown in FIG. 1, and an n-type collector region (1-10) for improving characteristics of elements in a high current region is then formed by selectively ion implanting to only an active region of the element.
In a subsequent step, a SiGe base layer (1-11) is selectively grown on only an exposed portion in the collector region (1-10) and the polysilicon film (1-7), through the use of Gas Source Molecular Beam technique, and then a polysilicon film (1-12) is selectively grown on the remaining space, thereby accomplishing junction between the polysilicon film (1-7) for a base electrode and the SiGe base layer (1-11).
Accordingly, self-alignment between the collector and the base can be performed, since a parasitic capacitance region formed between the collector and the base is limited to only portion of a polysilicon thin film (1-12) without being defined by a photoresist.
Since, however, the parasitic capacitance region defined by the polysilicon thin film (1-12) is determined from a horizontal wet etching for the insulating film (1-6), resulting in reduced efficiency of the manufacturing process in regards to uniformity and reproduction thereof, thus entailing the fatal degradation of the performance of the device.
In addition, the prior art method has a disadvantage that since the low speed selective epitaxial growth method is applied twice during the growth of the SiGe base layer (1-11) and the poly-silicon thin film (1-12), and two thin films for example, the SiGe base (1-11) and the polysilicon (1-12), are used during the growth process thereof, resulting in a complicated manufacture process, and further, even if the polysilicon thin film (1-12) is slightly grown on the base layer (1-11), it causes the fatal degradation of element performance, thereby making it difficult to control the process thereof. Thus, the prior art method can hardly accomplish an effective manufacturing process.
Furthermore, as shown in FIG. 1, the prior art method has a shortcoming in that a trench structure for isolating elements is not deeply formed so as to prevent the collector junction portion (1-4) from contacting between elements via the--collector thin film (1-3) on the n+ buried Si collector layer (1-2) formed at the entire surface of a substrate, resulting in a larger space requirement to fill with the insulating thin film (1-5), thus entailing a bulkier element.
FIG. 2 shows a cross-sectional view of Si/SiGe heterojunction bipolar element manufactured by another prior art method, after the growth of a base thin film.
In the prior art shown in FIG. 2, both the base and the collector thin films are grown through the use of selective epitaxial growth method in contrast with the trench structure previously described, to thereby provide simplified and integrated manufacture processes.
In FIG. 2, an n'-type collector (2--2) is first formed on a p-type substrate (2-1), and then an insulating thin film (2-3) and a polysilicon thin film (2-4) for a base electrode are subsequently deposited thereon, and thereafter, a base electrode region is defined by a photoresist mask and etching the poly-silicon thin film (24).
Then, an insulating thin film (2-5) is deposited thereon, and then the photoresist mask, the insulating thin film (2-5), the poly-silicon thin film (2-4) and the insulating thin film (2-3) are defined as an active region by an etching process.
Subsequently, an n-type silicon layer (2-6) for a collector, a SiGe layer (2-7) for a base, and a silicon layer (2-8) for an emitter are grown through the application of an impurity thereon.
During the growth of the layers, (2-6), (2-7), and (2-8), as shown in FIG. 2, poly-crystal or amorphous silicon thin films, (2-6-1), -(2-7-1) and (2-8-1), are formed at each side thereof, respectively.
Thereafter, a silicide thin film (2-9) for a collector metal junction is formed, and a metal electrode (2-10) is then deposited to thereby obtain an element.
However, the above element suffers from a drawback in that, since a current path passing via a sequence of the amorphous silicon thin films, (2-8-1), (2-7-1) and (2-6-1) occurs, it is prcne to a short between the collector and the emitter.
Likewise, current paths passing through a sequence of the amorphous silicon thin films (2-8-1), (2-7-1), (2-61) and the n-type Si thin film (2-6), or a series of the thin film (2-6), the thin films, (2-7-1) and (2-6-1) may occur, thereby causing emitter-base and a base-collector shorts.